HMC News – December 2025
Tule fog is a dense, ground-hugging fog that forms in California’s Central Valley during the late fall and winter months, especially after winter rains. It develops when moist air becomes trapped under a layer of cool, stagnant air, creating extremely low visibility that can reduce sight to just a few feet. On foggy days the diurnal temperatures stay low all day. There is a much smaller range between the high and low temperature then we see on a typical winter’s day. This helps with chill hours because it allows for more consecutive hours at lower temperatures, but the fog traps heat near the ground and by releases heat as it forms which keeps us from reaching freezing temperatures.
The California table grape season is winding down to its end. Northern Peruvian shipments have landed in both the East and West ports in a small way. Imports will continue to pick up as Chilean fruit begins to arrive early next month. Cold, foggy mornings continue to be the norm in the San Joaquin Valley. In the vineyards we have begun prepruning. Not long ago, prepruning was done by hand, and was long tedious work. In the last few years, we have used a prepruner. This machine is specially designed to remove the previous year’s growth from our vineyards. We then come into the fields at a later date and hand prune the lead vine down to approximately 25-30 spurs depending upon the variety. This is done every year once a vine is established (which takes approximately 3 years).
This month, daytime high temperatures in the San Joaquin Valley have been lower than usual due to the prolonged overcast conditions of recent weeks. Normally, clear winter nights allow temperatures to drop sharply at night and warm up during the day, creating a 20-30 degree shift in high and low temperatures. This winter, persistent cloud cover has trapped heat near the surface, creating lows that are higher than normal, and highs that are lower than normal, giving us a full 24 hours of chill time as opposed to the normal 12 – 15 hours we typically see this time of year. As you can see in the chart, we started December with the lowest number of chill hours over the last 5 years and have caught up rapidly due to the current weather pattern.
